This invention relates generally to the field of air pollution control, and is particularly directed to a system to remove condensed scrubbing products from a gas that has been scrubbed of sulfiur dioxide and other acid-forming gases.
Over the past several decades the control of air pollution has become a priority concern of society. The United States and other countries have developed elaborate regulatory programs aimed at requiring factories and other major sources of air pollution to install the best available control technology (BACT) for removing contaminants from gaseous effluent streams released into the atmosphere. The standards for air pollution control are becoming increasingly stringent, so that there is a constant demand for ever more effective pollution control technologies. In addition the operating costs of running pollution control equipment can be substantial, so there is also a constant demand for more energy efficient technologies.
Two well known types of devices to remove common particulates from a gaseous effluent stream are electrostatic precipitators (ESPs) and fabric filter baghouse (FFB) collectors. ESPs are generally recognized as being capable of a high particle collection efficiency of fine particles when the particles have the proper electrical resistivity. FFBs are also generally recognized as being capable of a high particle collection efficiency of fine particles when the particles have the proper characteristics. Typically, pollutant gases are conditioned such that the particulates can be more efficiently filtered by ESPs or FFBs.
However, ESPs and FFBs do not remove many gaseous species. Certain species of acid-forming gases can penetrate conventional particulate collection devices such as ESPs or FFBs resulting in emissions violations, causing downstream corrosion of components, and contributing to visible condensed particles which form typically after exiting the stack. In particular, ESPs and FFBs do not remove sulfur dioxide, hydrogen chloride, or other gases that form acids when dissolved in water.
The acid-forming gases that may be released from an air pollution control system with ESPs or FFBs may violate pollution control standards, may contribute to the xe2x80x9cdetached plumexe2x80x9d phenomena and may contribute to acid rain. Consequently, pollution control systems for applications generating large quantities of acid forming gases, such as coal-fired power plants, often require additional means to scrub acid forming gases from the effluent stream. Other applications where the emission of acid forming gases may be a problem include small coal-fired boilers, municipal waste incinerators, and medical waste incinerators.
Acid forming gases may contribute to the formation of visible plumes of effluent that violate opacity regulations even thought the total quantity of acid forming gases released into the atmosphere is comparatively minor. For example, one air pollution control problem for cement plants is the formation of a detached plume. Experimental studies have identified the detached plumes as being comprised primarily of ammonium sulfate and ammonium chloride particulates that form in the effluent flow and condense as the emissions from the stack cool in the atmosphere a distance from the stack. The ammonium particulates are in a size range of approximately one micron, which is a size that is efficient at scattering and reflecting light. The small size of the particulates and their high scattering efficiency means that an optically opaque plume can be comprised of a comparatively small total mass of ammonium sulfate and ammnonium chloride particulates. These detached plumes consist of a fine white plume that may last for hours or days depending on plant and atmospheric conditions. The plumes are highly noticeable and may violate pollution control regulations for opacity, and are thus a potentially serious problem.
Modern cement plants typically use ESPs or FFBs to reduce particulate emissions. However, these particulate filters do not remove the component chemicals that form detached plunes. In particular, ESPs and FFBs do not capture gaseous sulfur dioxide and gaseous hydrogen chloride which contribute to the chemical reactions that form detached plumes. In principle, an additional chemical scrubber could be added immediately after an ESP or FFB. However, conventional methods to scrub acid forming gases are typically expensive and inconsistent with the economic operation of an energy efficient plant. For example, conventional wet scrubbers, which commonly use spray droplet sizes greater than 1000 microns, typically use 10-100 gallons per minute of scrubbing liquid to scrub 1000 cubic feet per minute of effluent gases (1-10 kilograms of liquid per kilogram of gas). Consequently, the consumption of water, scrubbing chemicals, and energy is large for conventional liquid scrubbers. In addition, conventional liquid scrubbers have highi maintenance costs.
Modern energy efficient cement plants typically use two methods to cool and condition hot process gases before they enter an ESP or FFB. In a first stage of cooling, a gas conditioning tower (GCT) uses a spray of water to cool and condition the gaseous effluent. A second stage of cooling and conditioning is performed when the feed mill is operational by contacting the partially-cooled effluent with the cool wet limestone of the feed mill supplying fresh meal to the kiln.
Typically, the hot cement kiln gases must be cooled to approximately 150xc2x0 C. to provide efficient collection in an ESP or cooled to approximately 180xc2x0 C. to protect a FFB from overheating. When the feed mill is on, the gas conditioning tower typically must only cool the gaseous effluent to around 250xc2x0 C. In the mill-on state, the effluent is further cooled to between 100xc2x0 C. to 150xc2x0 C. as a result of contact with the feed meal. However, when the feed mill is shut off, the cooling tower must provide all of the cooling. There are thus two distinct operational states of the cement plant, corresponding to a mill-on and a milI-off condition.
One solution to the problem of sulfur dioxide emissions from a cement plant is to inject a lime slurry into the spray used to cool the GCT. A lime slurry is a desirable scrubbing material in a cement plant because lime is chemically compatible with other chemical constituents of cement, such that cement will not be contaminated if small quantities of lime enter the feed meal subsequent to the GCT. The chemicals in the lime slurry react with sulfur dioxide to produce thermally stable salts, thereby reducing sulfur dioxide emissions. However, conventional approaches to injecting a lime slurry into the cooling water of a GCT have low collection efficiencies and consume large quantities of lime slurry (see, e.g., Satish H. Sheth, xe2x80x9cSO2 Emissions History and Scrubbing Historyxe2x80x9d, pp. 213-217, 33rd IEEE Cement Industry Conference, Mexico City, Mexico May 1991). Conventional approaches to reducing sulfur dioxide emissions from cement plants are not consistent wit high sulfur dioxide collection efficiencies (e.g., greater than about 50%) with low molar ratios (e.g., less than about 3) of calcium hydroxide to sulfur dioxide. In some applications the maximum achievable sulfur dioxide collection efficiency may be unacceptably low, even at extremely high molar ratios of calcium hydroxide to sulfur dioxide. The large lime consumption required in conventional lime slurry injection schemes increases the operating cost and exacerbates the problems of the clogging and plugging of valves and nozzles. Additionally, the cost of the lime is further increased in conventional lime slurry injection schemes utilizing filtered slaked lime because only a fraction of the slaked lime ends up in the filtered slurry.
There are several factors that have previously made the use of a lime slurry in a GCT spray an inefficient and impractical means to scrub acid-foming gases in conventional pollution control systems used in cement plants. Some of these factors tend to limit the collection efficiency. Generally, the total quantityof spray in a GCT is kept as low as possible to reduce energy costs and water consumption and to minimize the temperature drop through the tower. This reduces the total volume of spray droplets that can absorb gases. The evaporative lifetime of spray droplets in a GCT is also short, typically a few seconds, which reduces the time available for gases to be absorbed. Also, the chemical reaction rates of a lime slurry with the absorbed acid-forming gases may be comparatively slow. In conventional approaches the acid forming gases are not adsorbed and converted into salts in the spray droplets at a fast enough rate to efficiently collect sulfur dioxide using low molar ratios of calcium to sulfur.
Other factors that may make use of a lime slurry in a GCT spray inefficient or impractical are the need to limit or eliminate from the final product alkali material or the salts resulting from the reaction of the lime with acid forming gases, and the need to minimize the deposition of lime-reacted solids from accumulating within a plant. Alkali materials and salts have low volatility and thus accumulate with time, caking up within plant process equipment and presenting corrosion or plugging problems. One conventional method of reducing unwanted alkali deposits is to modify the plant to include an alkali bypass to condense these materials. An alkali bypass, for example, is installed at the kiln outlet to quench some of the gases and collect the alkali materials. Although this is effective, it tends to consume large amounts of energy, which decreases the overall system efficiency, and also removes some of the economically valuable kiln dust along with the alkali condensate.
There are also other applications, such as small coal-fired boilers, municipal waste incinerators, and medical waste incinerators, where the use of a scrubbing spray comprised of a lime slurry is desirable. Lime is a comparatively safe scrubbing chemical whose chemical components are consistent with many different pollution control systems. However, scrubbing acid-forming gases with a spray containing a lime slurry may also be prohibitively expensive in these applications because of slow reaction dynamics and a low reaction efficiency. The inventors believe that there are numerous potential applications for a scrubbing spray comprised of a lime slurry that are rendered impractical because of the poor collection efficiency and large quantities of lime required using conventional approaches.
What is desired is an apparatus and method that permits acid forming gases to be efficiently and economically scrubbed using a spray composed of a cooling liquid and an alkaline chemical. In addition, an apparatus and method is needed to selectively remove both unreacted chemical and reaction products of the scrubbing process from the scrubbed gases.
The present invention generally comprises an apparatus to create a spray of fine droplets composed of a cooling liquid and an alkali chemical. A two-fluid nozzle is used to create spray droplets preferably having a mean diameter less than about 200 microns. A source of alkaline chemical is injected at a controlled rate into the liquid before it is sprayed. In the case of alkaline earth chemicals, the chemicals must have finely ground hydrated lime particles with a mean diameter less than about 25 microns. Preferably, the mean diameter of these particles is between one to ten microns.
One aspect of the present invention is that the mean particle diameter may be selected such that the particles rapidly release chemical into the spray droplets and substantially dissolve during the evaporative lifetime of a droplet. In one embodiment the source of alkaline chemical is hydrated lime particles. Still another aspect of the present invention is that the mean diameter of the spray droplets may be controlled to increase the surface-tovolume ratio of spray droplets, increasing the absorption rate of sulfur dioxide and other acid-forming gases. Yet another aspect of the present invention is that the characteristics of a spray composed of a liquid and lime slurry may be selected to achieve an efficient sulfur dioxide scrubbing function in a chamber, such as a gas cooling tower.
Another aspect of the present invention is to provide an apparatus and method for the removal of contaminants from a gas stream by reacting the contamninants with an alkali-containing spray to produce particles that can be efficiently removed from the gas. In one embodiment, the amount of hydrated lime or other alkali in each droplet is chosen to result in reaction products having a size and density that allows for efficient separation In another embodiment, the reaction product diameter is sufficiently different from the size of other entrained dust to allow preferential removal of the reaction products.